1. Field of the Invention
This patent specification relates to an image forming method and apparatus capable of reducing downtime of an image forming apparatus while maintaining toner density stability.
2. Description of the Related Art
In a background image forming apparatus such as a copier and a printer, image forming conditions need to be controlled to maintain toner density of images formed by the background image forming apparatus at a desirable level. The image forming conditions are controlled to appropriately adjust a toner amount by, for example, forming test toner patterns on a non-image area of a toner-image carrying member and detecting an amount of toner adhered to the test toner patterns. Particularly in a full-color image forming apparatus, such detection needs to cover a wide range from relatively high-density parts to relatively low-density parts of an image to maintain density gradation and color reproductivity of the image at desirable levels.
To obtain a toner gradation sequence, a plurality of toner image patterns need to be formed for detection of a wide range from relatively high-density parts to relatively low-density parts of an image. Generally, a plurality of toner image patterns are formed by applying different development bias voltages, different exposure energies, and so forth to a surface of an image carrying member in a rotation direction thereof. Then, the plurality of toner image patterns sequentially reach a detecting position of a toner density sensor, and the toner density sensor sequentially detects toner density values of the respective toner image patterns. In this case, the detection takes a relatively long time and thus may not be performed frequently. Therefore, the detection is performed after printing a predetermined number of sheets or at predetermined time intervals, for example.
According to a two-component development system, toner is mainly consumed as an image forming operation continues to be performed. Therefore, toner is replenished in a development device and is charged by friction together with carrier by using a conveying screw or the like. Then, the toner is conveyed onto a surface of a magnet roller (i.e., a development roller), and toner images are developed thereon. In a small-size image forming apparatus using this system, however, an amount of carrier is limited. As a result, if toner is replenished in the development device in a relatively short time, concentration distribution of toner supplied onto the surface of the magnet roller may temporarily become uneven.
Therefore, in the background image forming apparatus, toner image patterns are formed after having equalized a concentration distribution of toner by rotating a conveying screw provided at an upstream side of a magnet roller to mix and convey the toner and carrier.
For example, if a development device is replenished with its maximum replenishment amount of toner, and if a predetermined time period is set as a time period required for equalizing the concentration distribution of toner, toner image patterns are formed always after the conveying screw has been rotated for the predetermined time period. An amount of toner replenished, however, corresponds to an amount of toner consumed, which relates to a pixel count of images formed on a printed page, for example. Therefore, the amount of toner replenished is not necessarily the maximum replenishment amount. As a result, if the amount of toner replenished in a toner replenishment is smaller than the maximum replenishment amount, the time required for equalizing the concentration distribution of toner which was made uneven by the toner replenishment may be shorter than the above predetermined time period. As in this case, if the time period required for equalizing the concentration distribution of toner is set at a predetermined value, unnecessary downtime of the image forming apparatus results.
To attempt to resolve this problem, there is a technique using a video counter for counting and summing up image data values of images developed by a plurality of development devices, and a developer density detection device for summing up the image data values and detecting information of toner consumption in the developer. In this technique, toner is replenished in accordance with a toner replenishment signal which is based on a signal output from the developer density detection device, and a speed of driving the mixing member can be varied according to a value of the video counter. This technique, however, is to simply optimize the speed of driving the mixing member and is not based on an idea to reduce the downtime of the image forming apparatus while maintaining toner density detection stability.